1. Field of the Invention
The present invention relates to an electrically driven steering apparatus which provides its steering mechanism with an assist force by a motor and more particularly to an electrically driven steering apparatus capable of detecting a steering angle using a rotation angle sensor.
2. Description of the Related Art
Conventionally, an electrically driven steering apparatus which reduces a steering force through a steering wheel by providing the steering mechanism connected to a steering shaft with an assist force by a motor has been well known. Because in such an electrically drive steering apparatus, the steering wheel is rotated within a finite revolution number of more than one turn in the right or left direction, at what degree the steering wheel is located in the right or left direction from a neutral position of the steering wheel, which is determined in a condition that a vehicle travels straight, is recognized as an absolute position by a sensor. If this absolute position is used by the rotation angle sensor provided on the steering wheel, for example when a half rotation state (180°) is detected, it is impossible to specify whether the steering wheel is rotated by a half turn to the right side (+180°) from the neutral position or rotated by one and a half turns to the right side (+540°) or rotated by a half turn to the left side (−180°) or rotated by one and a half turns to the left (−540°). Thus, the absolute position is determined by incrementing (+360° when it is rotated a single turn to the right) or decrementing (−360° when it is rotated a single turn to the left) the rotation number of the steering wheel each time when the steering wheel is rotated by a single turn.
At the time of ignition ON, the above-described method is not capable of detecting the absolute position because the rotation number of the steering wheel is not clear. In this case, the absolute position is detected by using plural rotation angle sensors. That is, attention is paid to that the motor for generating the assist force utilizes a resolver (rotation angle sensor) as a means for detecting a motor rotation position while another resolver is used as a means also for detecting the steering torque of the steering wheel and then, the absolute position of the steering wheel can be detected by computing signals outputted from plural resolvers and having linearity to the rotation angle. This computation of the absolute position by arithmetic operation is carried out at the time of ignition ON and after that, the absolute position is obtained with the rotation angle sensor which increments or decrements the rotation number when the steering wheel is rotated by a single turn.
[Prior Art]
JP2003-75109 A is incorporated herein by reference.
However, if the rotation angle sensor for obtaining the absolute position (steering angle) is in trouble, not only the steering assistance by the motor is stopped but also detection of the steering angle is disabled immediately, so that, for example, vehicle stability control (VSC) which is achieved by using the steering angle is made inactive either.